Tire balance, loose parts in the front end, perhaps a driveline issue.
I'm with fullaluv, sounds most like tire balance to me. If it started suddenly it could be you lost a wheel weight or someone knocked one off for you. It could also be a tire about to separate. I just had one do it to me and it starting vibrating for about a week before just letting go. If it has been going for a while then you could have alignment issues wearing the tires uneven and creating out of balance problems. Either way, easiest to just take it to a tire shop and have them balance the tires. Usually if you go to a shop that sells the brand of tires you have on you can get them to do it for free.
To check out the front end, have someone wiggle the steering wheel back and forth and crawl under and see where things are moving with slack. Also jack up each front wheel and try to wiggle it. Top to bottom and then side to side. If it moves/knocks both ways it is likely wheel bearings, if it just moves top to bottom it is likely a ball joint. If it just goes a little side to side it would be in the steering linkage somewhere. Worst wear point on these is the idler arm bushing, its the thing on the passenger frame rail in the same position as the steering gearbox is on the drivers side.
The driveline is a bit tougher to diagnose. Crawl under and try to wiggle it around. There shouldn't be any slop in the u-joints or at the rear axle. If it is bent or out of balance it is really hard to check in the driveway. You'd have to find a driveline shop.
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
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WOW! I really like my truck a lot better now! 
Got a new windshield paid for and will be installed tomorrow. $165.00